delphi默认消息拦截

Code
unit uWei;
interface
uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;
type
  TForm2 = class(TForm)
    Button1: TButton;
        procedure aa(var msg:twmnchittest);message wm_nchittest;  //消息处理过程序声明
  private
    { Private declarations }
  public
    { Public declarations }
  end;
var
  Form2: TForm2;
implementation
{$R *.dfm}
    procedure tform2.aa(var msg:twmnchittest);
    begin
      inherited;
      defaulthandler(msg);
      if msg.Result=htclient then
        msg.Result:=htcaption;
    end;
end.